1. Field of the Invention
This invention generally relates to laser material processing and, more particularly, relates to methods and systems for high-speed, precise trimming utilizing a laser and scan lens for use therein.
2. Background Art
Laser trimming has been a part of manufacturing process in semiconductor and microelectronics industries for more than 30 years. One of the challenges is always to reduce the resistance drift after the trimming process. Post-trim stability is extremely important since the purpose of trimming in the first place is to increase device accuracy. If the device later drifts out of specifications, nothing has been gained. It has been known that trim induced instability or long-term drift arises from the heat-affected zone (HAZ) along the laser cut edges and any residual material in the cut itself. The laser trim itself causes heating and melting of film material near the trim. This heating causes a change in the sheet resistance, the temperature coefficient of the resistance (TCR), and the aging characteristics in the zone adjacent to the trim. Resistor material that has been heated to a very high temperature, but not vaporized, will have its electrical characteristics altered somewhat. The electrical resistance of these regions tends to increase over time before becoming stabilized.
The magnitude of the change is primarily dependent on the resistor material as well as the laser processing parameters. With the current laser technology, this potential instability due to the heat-affected zone along the laser cut edges remains. This is inherent in the trim process and it cannot be eliminated. The use of link cut geometry may be one solution since once the link is severed, little or no current flows in the unstable region. But links with reasonable resolution require a disproportionate share of the device real estate and are only used for simple trims such as op-amp offset voltage or in conjunction with a continuous trim in a course/fine arrangement. Simply designing larger resistors is another way to reduce the instability since this allows the current to spread over a larger area and the unstable portion becomes a smaller percentage of the total. However, this will squander precious real estate since cutting drift in half requires doubling the resistor size. Similarly, making the laser spot smaller reduces the size of the unstable region relative to the overall current carrying area resulting in an improvement in overall stability. This is limited, however, by the choice of the laser wavelength, of optics, and by various practicalities such as reduced depth of focus, less working distance, and material re-deposition in the case of thick films.
Traditionally, a Nd:YAG laser with wavelength at 1 micron is used for trimming of chip resistors. As the sizes of resistors get smaller, the substrates thinner, and tolerances tighter, this wavelength hits its fundamental limitations in terms of trimming kerf width, heat-affected zone (i.e., HAZ) and, therefore the drift of TCR and Resistance, R.
It is well known that shorter wavelengths can provide smaller optical spot size. It is also well known that the absorption of the film materials at shorter wavelength is higher. Therefore, the use of lasers with wavelengths shorter than the traditional 1 micron have the advantages of smaller kerf width that allows smaller features to be trimmed, and of smaller HAZ that leads to much less TCR drift and R drift.
As disclosed in the following U.S. Pat. Nos. 5,087,987; 5,111,325; 5,404,247; 5,633,736; 5,835,280; 5,838,355; 5,969,877; 6,031,561; 6,294,778; and 6,462,306, those skilled in the art of lens design will appreciate the complexities of scan lenses designed for multiple wavelengths.
Many design parameters are considered and various design trade-offs such as spot size, field size, scan angle, scan aperture, telecentricity, and working distance are used to achieve a laser scan lens design solution for trimming applications. In order to achieve a small spot over a large scan field, as preferred for high speed processing of fine structures over large areas, the scan lens must be able to focus a collimated input beam and image a diffraction limited laser spot over the entire field. The spot must be sufficiently round and uniform across the field to produce uniform trim cuts within the field. The lens must also provide adequate viewing resolution to image a selected target area for calibration and process monitoring. For through-the-lens viewing, light is collected from the illuminated field, collimated by the scan lens, and imaged onto a detector using auxiliary on-axis optics. By utilizing a different wavelength region for target viewing and an achromatized scan lens, efficient beam combining and splitting is possible using conventional dichroic optical elements. Within the viewing channel, good lateral and axial color correction is required, however small amounts of lateral color between the viewing and laser channels can be accommodated in the scan system and small amounts of axial color between the viewing and laser channels can be accommodated with focus adjustments in the field or in auxiliary optics. With a two mirror scan head, for example a galvanometer scan head when pupil correcting optics are not used, the scan lens must accommodate the pupil shift resulting from the separation between the two scan mirrors.
Relative lens capability can be determined by dividing the field size by the imaged spot size to find the number of spots per field. Conventional achromatized scan lenses for laser trimming, for example, the objective used in the GSI Lumonics W670 trim system for thick film trimming with a laser wavelength of 1.064 microns, produces a 30 micron spot over a 100 mm square field and images the target with conventional white light sources and auxiliary camera optics to a monochrome CCD camera. The W670 system is capable of about 4667 laser spots over the field diagonal. Lenses in system used for thin film trimming have smaller field sizes and smaller spot sizes. For example, the scan lens used in the GSI Lumonics W678 trim system, also with white light viewing capability, has a 12 micron spot over a 50 mm field, or about 4167 spots. Yet another thin film scan lens with a laser wavelength of 1.047 microns is used in the GSI Lumonics M310 wafer trim system, has a 6.5 micron spot over a 1 cm sq telecentric field and is capable of about 2175 spots with IR LED illuminators with an emission band of about 860 nm to 900 nm for viewing.
To some extent, lenses or lens design forms intended for IR laser scanning, especially IR scan lenses with white light viewing, can be used or modified to other laser wavelengths, for example, with green lasers. Reducing the wavelength theoretically reduces the spot size proportionally. However, considering increased lens aberrations and manufacturing tolerances, this may not be achievable. For example, a green version of the W670 lens produce a spot of about 20 microns compared to 30 microns for the IR version, and the number of spots per field is increased from 4667 to about 7000.
Conversely, it has been found that lenses designed primarily to operate at a green laser wavelength with a viewing channel at a longer wavelength can be optimized to scan a second wavelength, for example 1.047 microns or 1.064 microns, producing a spot approximately scaled up by the wavelength.
The following exemplary U.S. patents are related to laser trimming methods and systems: U.S. Pat. Nos. 6,534,743; 6,510,605; 6,322,711; 6,281,471; 5,796,392; 4,901,052; 4,853,671; 4,647,899; 4,511,607; and 4,429,298.
U.S. Pat. No. 4,429,298 relates to many aspects of serpentine trimming. Basically, a serpentine resistor is formed with sequential plunge cuts and a final trim cut is made parallel to the resistor edge from the last plunge. It describes “progressively” making plunge cuts on a resistor alternately from one end, considers maximum and minimum plunge cut lengths, a resistance threshold of the plunge cuts for the trim cut, a faster cutting speed for plunge cuts, and a structured process flow with various resistance and cut length tests.
There is a continuing need for improved high-speed, micromachining such as precise trimming at all scales of operation, ranging from thick film circuits to wafer trimming.